Information regarding the mechanical activity of the heart may be of interest for both diagnosis and treatment of heart diseases. Such information may be useful for early diagnosis of a cardiac disease, for assessing a disease severity and for facilitating decision and planning regarding possible treatment.
In particular, information regarding the mechanical activity at different regions of the heart may be useful for various applications. For example, in applications such as artificial pacemaker implantation, particularly in the case of a biventricular pacing, it is advantageous to map the local mechanical activity of a patient's heart in order to determine optimal position for lead implantation. Artificial pacing provides electrical stimuli for causing cardiac contraction when the intrinsic myocardial electrical activity is impaired. A pacing system consists of a pulse generator and insulated electrode leads which carry the electrical impulses from the generator to the heart. The pulse generator is typically placed subcutaneously or submuscularly in the chest wall while the insulated leads are inserted transvenously to be attached to the heart. The procedure is typically facilitated by fluoroscopy which enables the physician or cardiologist to view the passage of the electrode lead. At the tip of each lead is an electrode, or several electrodes, that deliver the necessary electrical impulses to the specific location of the tip. The success of the pacing therapy depends to a great extent on the specific locations of the implanted leads, particularly in the case of a biventricular pacing.
A biventricular pacing, also known as cardiac resynchronization therapy (CRT), is an interventional procedure in which both the right ventricle (RV) and left ventricle (LV) of the heart are stimulated synchronously to improve the heart pumping efficiency. About 20 to 30 percent of patients with heart failure suffer from desynchronized contraction of the right and left ventricles (desynchrony) and need a biventricular pacing for pacing both sides of the heart to restore synchrony. While a standard pacemaker typically has two leads for stimulating the right ventricle and the right atrium, a biventricular pacemaker has an additional lead for pacing the left ventricle. As with a standard pacemaker, the first and the second wires are threaded through the veins to the right ventricle and to the right atrium, usually through the sub-clavian vein and/or cephalic vein, which are relatively easily accessed from the pocket under the skin. In a bi-ventricular pacemaker, a third wire is implanted in order to stimulate left ventricular wall contraction, in a more complicated procedure. The third wire passes through the right atrium into the coronary sinus (CS) and then placed through one of the CS branches to pace the left ventricle. The location of the implanted lead in the left ventricle is very crucial to the therapy efficiency. Indeed, about 30 percent of the time following a CRT implant the patient does not respond to the therapy (non-responders). A possible way to improve the response rate may be by placing the lead for pacing the left ventricle in the area that contracts last. Thus, a proper application of a biventricular pacing therapy would require detailed knowledge of the temporal and spatial characteristics of LV contraction on a patient-by-patient basis. Useful information may also be gained from comparing the temporal-spatial distributions of the mechanical and electrical activations. Such comparison may indicate whether the cause of dysfunction at a specific location is a defect in the heart's electrical conduction system or a scarred tissue. For example, if both electrical and mechanical activation are delayed with a similar latency, the problem is most likely in the electrical conductivity. In such a case, external pacing can improve synchronization of contraction. However, if the delay in mechanical activation is significantly longer than the delay in electrical activation, the problem is probably caused by mechanical injury of the tissue.
The CRT application is only an example of a cardiac procedure to which the invention is directed. Another exemplary cardiac procedure anticipated by the invention, for which an analysis of regional mechanical activity of the heart may be beneficial, is the investigation of coronary arteries. Narrowing of coronary arteries, i.e., a stenosis, can lead to reduced blood flow to a heart muscle, angina, and eventually to a heart attack. It is therefore important to identify stenotic arteries and to assess stenosis severity in order to decide if and which interventional treatment is required (e.g., balloon angioplasty and/or deployment of stents). Typically, the severity of the stenosis is determined by assessing the extent of the artery narrowing or by measuring blood pressure and flow gradient between proximal and distal parts of disease using FFR (Fractional flow reserve) techniques. Analysis of the regional or local mechanical activity of the heart, as provided by the present invention, and in particular assessing the extent of contractility of the wall at the region distal to the stenosis, may facilitate assessing of a stenosis severity. Similarly, analysis of the local mechanical activity of the heart may facilitate identification of necrosis and infarct areas.
A variety of volumetric imaging systems are presently available, such as ultrasound, computed tomography (CT) and magnetic resonance, which provide time-sequence volumetric data that may be analyzed to obtain spatial-temporal information of the heart mechanics. However, volumetric imaging modalities require expensive equipment which is not widespread and therefore are not conducted for all patients. Additionally, these systems are generally adapted for diagnosis and for planning interventional procedures but are not configured for real-time imaging during such procedures. On the other hand two-dimensional (2D) X-ray imaging methods by conventional fluoroscopy apparatus, such as a C-arm X-ray system, are routine diagnostic methods in most cardiology clinics and are typically deployed during interventional procedure, such as implantation of a pacemaker or a stent, to allow navigating and location the device.
There is therefore a need for a method and system for analyzing the mechanical activity of the heart in general, and of mapping local heart motion in particular, by using standard, widespread imaging equipment, such as fluoroscopy X-ray machine, which is available in most heart physiology and cardiology clinics.